1. Field of the Invention
The present invention relates to a radiographic image detecting device for detecting a radiographic image of an object and a control method thereof.
2. Description Related to the Prior Art
In a medical field, for example, an X-ray image capturing system using X-rays as a kind of radiation is known. The X-ray image capturing system is constituted of an X-ray generating device having an X-ray source for emitting the X-rays, and an X-ray image detecting device for detecting an X-ray image, which represents image information of an object, by receiving application of the X-rays that have been emitted from the X-ray source and transmitted through the object. The X-ray source is provided with a tube current for determining an X-ray dose per unit of time and a tube voltage for determining X-ray quality (energy spectrum), as an imaging condition. The imaging condition is determined whenever imaging is performed, in accordance with a body part to be imaged, the age, and the like of an examinee being the object. The X-ray source radiates the X-rays in accordance with the set imaging condition.
A type of X-ray image detecting device that uses an X-ray image detector (flat panel detector (FPD)), instead of a conventional X-ray film or imaging plate (IP), is in practical use (refer to Japanese Patent Laid-Open Publication No. 2010-214056). The FPD is constituted of a detection panel and a signal processing circuit. The detection panel has an imaging area in which a plurality of pixels each for accumulating signal charge in accordance with an X-ray irradiation amount are arranged in a matrix and signal lines connected to the pixels are laid out to read the signal charge from the pixels. The signal processing circuit reads the signal charge accumulated in each pixel as a voltage signal, and converts the read voltage signal into digital image data. Thus, the X-ray image detecting device using the FPD allows an observation of the X-ray image immediately following the taking of the X-ray image.
In the detection panel, each of the pixels provided in the imaging area is constituted of a photodiode, being a photoelectric conversion element, and a TFT (thin film transistor). A scintillator (phosphor) is provided on the imaging area to convert the X-rays into visible light. The TFT is a switching element that switches the operation of the pixel by the electrical connection and disconnection between the photodiode and the signal line. Upon turning off the TFT, the photodiode is disconnected from the signal line, and an accumulation operation for accumulating the signal charge to the photodiode is started. Upon turning on the TFT, the photodiode is connected to the signal line, and a readout operation for reading the signal charge from the photodiode through the TFT and the signal line is started.
In contrast to the X-ray film and the IP, the FPD requires a synchronous control in order to start the accumulation operation and the readout operation in synchronization with X-ray emission timing. As a synchronous control method, there are a method of communicating a synchronous signal between the X-ray generating device and the X-ray image detecting device, a method of determining emission start timing and emission stop timing by detecting an X-ray irradiation amount, and the like.
The Japanese Patent Laid-Open Publication No. 2010-214056 discloses that the X-ray irradiation amount is detected by an X-ray detection means provided in the imaging area of the FPD, and is used in various types of control. More specifically, taking advantage of the fact that X-ray irradiation causes variations in a bias current flowing through a bias line for applying a bias voltage to the photodiodes, the start and stop of X-ray emission are detected. The synchronization control, that is, the FPD is controlled in synchronization with the detection of the start and stop of X-ray emission is performed. The Japanese Patent Laid-Open Publication No. 2010-214056 also discloses an automatic exposure control (AEC) in which a total X-ray irradiation amount is measured based on the variations in the bias current, and the X-ray source stops emitting the X-rays when the total X-ray irradiation amount has reached a predetermined threshold value.
The bias lines are connected to all the pixels in the imaging area of the FPD. Thus, according to the FPD of the Japanese Patent Laid-Open Publication No. 2010-214056, the synchronization control and the automatic exposure control are performed by monitoring variations in the bias current using the entire imaging area as a detection area, and using one or more of divided sections into which the imaging area is divided as a detection area. In either of cases where the entire imaging area is assigned as the detection area and the divided section is assigned as the detection area, both the synchronization control and the automatic exposure control use the same detection area.
The object is positioned with respect to the imaging area of the FPD when taking the X-ray image. In a case where the object is smaller than the imaging area in size and is not opposed to the entire imaging area, like a hand or a foot, for example, the X-ray irradiation amount differs from place to place within the imaging area. To be more specific, a part of the imaging area that is not opposed to the object becomes a directly exposed area to which the X-rays are directly applied without passing through the object. The X-ray irradiation amount is large in the directly exposed area because of no absorption of the X-rays by the object. On the other hand, a part of the imaging area that is opposed to the object is irradiated with the X-rays passed through the object, so the X-ray irradiation amount is small due to absorption of the X-rays by the object.
Also, as shown in FIG. 12, in a graph in which a horizontal axis represents time and a vertical axis represents the X-ray irradiation amount, the X-ray irradiation amount gradually increases from the start of X-ray emission. The X-ray irradiation amount has reached a predetermined value, and then is kept at the value for a while until the stop of X-ray emission. As shown by a solid line in the drawing, the X-ray irradiation amount of the directly exposed area sharply rises and quickly reaches the threshold value because of no X-ray absorption by the object. On the other hand, more time is required by the X-ray irradiation amount of the area opposed to the object for reaching the threshold value, as shown by a broken line in the drawing, due to the X-ray absorption by the object. Thus, using the area opposed to the object as the detection area requires longer detection time, as compared with the case of using the directly exposed area as the detection area. This also causes high susceptibility to noise because of a low output value of a signal in accordance with the low X-ray irradiation amount, and low detection accuracy. In the synchronization control, it is desirable to be able to certainly detect a rise of the X-rays in a short time, so the directly exposed area is preferably used as the detection area for the synchronization control.
On the other hand, out of the imaging area of the FPD, the area opposed to the object is preferably used as the detection area in measuring the total X-ray irradiation amount in the automatic exposure control. This is because if the total X-ray irradiation amount of the directly exposed area is measured in the automatic exposure control, the measured total X-ray irradiation amount reaches the threshold value before a required amount of X-rays is applied to the object. This results in underexposure of the X-ray image.
According to the Japanese Patent Laid-Open Publication No. 2010-214056, the same detection area is used in both the synchronization control and the automatic exposure control. Accordingly, if most of the detection area is the directly exposed area, the underexposure may occur in the automatic exposure control, though the accuracy of the synchronization control is increased. On the contrary, if most of the detection area is opposed to the object, the accuracy of the synchronization control is reduced, though the accuracy of the automatic exposure control is increased.